Noise, and attenuation in long-haul optical line systems result in the deterioration of the transmitted signal, both as to its amplitude as well as its shape. Consequently, one of the fundamental requirements of nodal equipment in optical networks is the capability to regenerate and reshape the optical pulses. These functions are known as 2R, for regeneration and reshaping. Notwithstanding the plethora of claims by various companies to have implemented “all-optical” systems, presently retiming of the optical pulses is achieved by converting the incoming optical signal into an electrical signal. This is followed by full regeneration and reshaping of the electrical signal using Application Specific Integrated Circuits (ASICs). A laser source is then modulated using this fully regenerated and reshaped electrical signal. Such systems are termed OEO, or Optical-Electrical-Optical. However, there are certain drawbacks to converting an optical signal into an electrical one and back again. First, electrical processing of data signals is not transparent to bit rate and is format sensitive. Thus, an OEO system could not process an arbitrary incoming data signal; the bit rate, format and coding would need to be known a priori. Different bit rates require different ASICs to process them in the electrical domain. Second, there is a significant power loss in converting to the electrical domain, and a similar power loss in converting back again therefrom to the optical domain.
As optical networks become increasingly transparent, there is thus a need to regenerate the signal without resorting to OEO conversion of the signal. Such regeneration, if truly done all optically, is termed AO2R, for “all optical regeneration and reshaping.” This would free the network nodes from the limitations placed on signal processing by the electrical domain processing circuitry.
Future optical networking line systems will incorporate service signals at both 10 Gb/s as well as 40 Gb/s along with their associated Forward Error Corrected (FEC) overhead. Beyond that 80 Gb/s is just around the corner. The FEC rates related to, for example, 10 Gb/s data transport include the 64/63 coding for 10 Gb/s Ethernet, the 15/14 encoding of SONET-OC192 FEC and the strong-FEC rate of 12.25 Gb/s, as well as numerous potential coding schemes yet to be developed. Effectively, to support multiple FEC—and other coding related—protocols, an optical network node must be able to process numerous line rates.
In general, it is a useful function to be able to switch a signal that came in on one wavelength to output on another. This may arise when an input signal arriving from a client on one service wavelength is provisioned outbound on another. In conventional OEO 2R systems, it is a simple matter to switch an incoming signal to a different wavelength inasmuch as once the signal has been converted to the electrical domain, it is feasible to reconvert it to the optical domain on a different wavelength than the one it arrived on by using the electrical signal to drive a laser at a new different wavelength. The problem arises in achieving this functionality in an AO2R system, where the signal remains in the optical domain at its original wavelength.
What is needed therefore, is an AO2R system, that is truly all-optical, that is transparent to both bit rate and protocol or format, and that supports any wavelength in the carrier frequency range (wavelength range) of the modern telecommunications systems, the C and L wavelength bands, and that can convert an incoming signal to a different wavelength when it is output.